An seam-welded steel pipe is manufactured by a method in which a band steel is deformed by a series of rolls so as to have a circular section with an opening and opposing edges of the band steel are heated and welded by use of electric resistance. However, it is well-known that quality of a final product is considerably dependent on quality of the welded portion.
A so-called welding angle between the both heated edges of the band steel fused by a pressing operation of squeeze rolls is included in many factors relevant to the quality of the welded portion. Generally, as the welding angle becomes stable and large, it is easy to form a welding environment that a welding operation at the merging point becomes stable and various welding defects hardly arise.
In order to stabilize the welding angle and to securely obtain a necessary size, there is known a method in which a fin roll having a prescribed width is provided between final fin pass rolls and the squeeze rolls and inserted between the opposing edges of the band steel.
Patent Document 1: Japanese Patent Publication. No. 6-277751A
Patent Document 2: Japanese Patent Publication No. 5-16952B
Patent Document 3: Japanese Patent Publication No. 61-034908B